Method for controlling iron content of a zinc phosphating bath

ABSTRACT

A method of controlling the iron content of a zinc phosphating process. The processing bath is continuously aerated and agitated to precipitate iron compounds out of solution and maintain the precipitate in suspension. The bath is constantly recirculated through a series of settling tanks or a single compartmentalized tank wherein the precipitate is settled out of solution and from which relatively clarified solution is discharged back to the processing bath. Periodically iron bearing sediment is removed from the settling tanks for disposal as solid waste.

United States Patent [191 Hill [451 Apr. 1, 1975 METHOD FOR CONTROLLINGIRON CONTENT OF A ZINC PHOSPHATING BATH [75] Inventor: Edward AlexanderHill, Scotch [52] U.S. Cl. 148/6.15 Z, 134/13, 117/102 A [51] Int. ClC23c l/00, C23f 5/02, B05c 11/10 [58] Field of Search 118/600, 602, 603,610,

118/612; 117/102 A; 156/19; 134/13, 10; 148/615 R, 6.15 Z

[56] References Cited UNITED STATES PATENTS 1,146,071 7/1915 Hoffman134/13 2,132,883 10/1938 Romig 148/615 Z 3,100,727 8/1963 Labergere134/13 3,144,361 8/1964 Klinghoffer 148/615 Z X 3,401,065 9/1968Steinbrecher et a1. 148/6.15 Z

FOREIGN PATENTS OR APPLICATIONS 679,869 9/1952 United Kingdom 148/6.15 R

Primary Examiner-John D. Welsh Attorney, Agent, or Firm.lames R. OConnor[57] ABSTRACT A method of controlling the iron content of a zincphosphating process. The processing bath is continuously aerated andagitated to precipitate iron compounds out of solution and maintain theprecipitate in suspension. The bath is constantly recirculated through aseries of settling tanks or a single compartmentalized tank wherein theprecipitate is settled out of solution and from which relativelyclarified solution is discharged back to the processing bath.Periodically iron bearing sediment is removed from the settling tanksfor disposal as solid waste.

7 Claims, 1 Drawing Figure METHOD FOR CONTROLLING IRON CONTENT OF A ZINCPHOSPHATING BATH BACKGROUND OF THE INVENTION 1. Field of the InventionThe invention relates to immersion-type zinc phosphating systems andparticularly to methods for controlling iron content and sludgeaccumulation in the processing bath tanks of such systems.

2. Description of the Prior Art In applying zinc phosphate coating tosteel parts in immersion-type phosphating systems, the dissolved ironcontent of the phosphating bath solution must be controlled to preventretardation of the deposition of zinc phosphate in the coating process,i.e., to maintain the efficiency of an effectively monitored bath.

One past, and still too prevalent, practice has been to frequently shutdown the system and dump the phosphating bath which has reached anunacceptable dissolved iron concentration level, clean the processingtank to remove accumulated sludge, pour a new bath, and then reactivatethe system. The obvious disadvantages of this method are excessiveequipment down time, nonproductive labor expenditures, substantiallyincreased chemical consumption, and variations in the quality anduniformity of the phosphate coating on work being processed. Inaddition, the dumping of contaminated baths severely taxes the digestivecapacities of local sewage treatment facilities and provides for amassive injection of pollutants where sumps continue to be draineddirectly into watercourses.

A somewhat more economical, but less than optimum, method of control hasbeen to precipitate dissolved iron out of the bath by such means assupplying chemical additives to the bath or periodic batch aeration. Theiron precipitate settles and builds up as sludge on the sides and baseof the processing tank and periodically the system is shut down topermit scraping and removal of the accumulated sludge from the tank.Frequently, the phosphating bath is temporarily pumped to a storage tankwhile cleaning of the processing tank is carried out with the bath beinglater pumped back into the processing tank and brought up to a specifiedoperating level. While the latter procedures are certainly preferable tothat first mentioned from the aspects of pollution control and reductionof zinc phosphate con sumption, they nevertheless require periodicequipment shutdown, expenditures for nonproductive labor, expendituresfor chemical additives to precipitate the iron, expenditures for storagetanks and pumping equipment if the bath is to be temporarily drained andreconstituted after tank cleaning, expenditures for batch aerationequipment if that method of precipitating the iron is to be employed inlieu of chemical precipitation, and possible expenditures for one ormore standby systems to be utilized while a primary system is beingbatch aerated and desludged. Further, chemical additives to promote ironprecipitation frequently adversely affect the quality of zinc phosphatecoatings.

Iron precipitation combined with continuous decanting and replacement ofthe bath solution can be employed to extend the time interval betweenshutdowns for complete processing tank cleaning and desludging.Decanting procedures, however, have proved costly regarding chemicalconsumption and present a pollution problem which, although it is not asacute as batch dumping, recommends against this method of iron control.

Those familiar with the art will readily appreciate the difficulty inachieving uniformity of phosphate coating when the dissolved ironcontent of an operating bath is constantly varying, as it is when theaforementioned methodsof iron content control are employed. Since thephosphating chemical requirements change relative to the change in ironcontent, i.e., a higher iron level must be compensated for or offset bya higher phosphate chemical level, extremely close monitoring isnecessary to minimize bath fluctuation and maintain a uniform operatinglevel.

The present invention is seen to provide a method and apparatus for ironcontrol and waste disposal which represents a marked improvement overthe procedures mentioned above, without materially increasing thecapital expenditures for equipment necessary to implement the method andat the same time improving the quality of zinc phosphate coatings. Themerits of the inventions contribution to the art will become clearlyevident to one who proceeds to a consideration of thedetaileddescription of the invention which follows.

SUMMARY OF THE INVENTION To control theiron content of an immersion-typephosphating system, the phosphating bath is maintained in a constantstate of turbulence for surface aeration and suspension of ironprecipitate. A turbulence inducing mechanical mixer is located in theprocessing tank so as to induce a particularly strong turbulent flow ofbath solution over the base and upwardly along the sides of the tank andpast the bath heaters and around and through the work processing drums.The bath is constantly circulated by a pumping system from theprocessing tank through a series of settling tanks, or a singlecompartmentalized settling tank and back to the processing tank. Air, orpure oxygen, can be introduced into the bath solution at selectedlocations, advantageously at locations of maximum turbulence, to wit,adjacent the mechanical mixer and the outlet from the processing tank tothe pumping system, whereby the air or oxygen is driven or drawn intodispersion in the bath for maximum precipitation of dissolved iron.During passage of the bath solution through the settling tanks whereinthe flow is relatively quiescent, the iron precipitate settles to thebottom of said tanks and the latter are periodically purged of ironbearing sediment which is disposed of as solid waste. Substantiallyclarified solution is discharged from the most downstream of thesettling tanks back to the processing tank. The constant turbulent flowpath in the bath directs the fresh or clarified solution throughout thearea of the bath in which work is being processed and improves theuniformity of zinc phosphate deposition on the work. Optionally, anauxiliary tank or separate compartment may be located upstream of thesettling tanks or compartments and the auxiliary tank may be equippedwith a mechanical mixer and means for introducing additional air oroxygen into the solution to further enhance iron precipitation prior toentry of circulating solution into the settling tanks or compartments.

BRIEF DESCRIPTION OF THE DRAWING The drawing depicts a schematic view ofan apparatus for effecting iron content control of a zinc phosphatingbath according to the improved method of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT In the accompanying schematicdrawing of the apparatus, there is depicted a main processing tank 2filled substantially to its top with a zinc phosphating bath 4.Perforated rotatable drums 6 for receiving steel parts to be processedare immersed in the bath as are platetype heat exchangers 8 locatedadjacent to but spaced from the sides of the tank for maintaining thebath at a prescribed operating temperature and directing the flow ofbath solution as is described in detail hereinafter. A mechanical mixingdevice 10 includes a turbinetype impeller 12 immersed in the bath anddisposed generally centrally adjacent the bottom of the tank 2. Theimpeller is adapted to be rotatably driven through connecting shaft 14by a driving mechanism 16 suitably mounted at the top of the tank. Twopipes for conducting air or pure oxygen to the phosphating bath areconnected to the tank 2. The first pipe 18 has its outlet 20 locateddirectly beneath the impeller 12. The outlet 22 of the second pipe 24 islocated in the lower region of the left end of the tank.

An intake pipe 26 disposed at the left end of the tank 2 leads to acirculating pump 28 which is in turn connected to transfer pipe 30leading to the first or most upstream of a series of communicatingauxiliary tanks or compartments 32. Each of the tanks or compartments 32has a funnel-shaped bottom section 34 terminating in discharge ducts 36normally closed by valves 38. Each of the tanks or compartments 32 alsoincludes a transverse baffle 40 disposed substantially adjacent theinlet end of each tank. Return transfer pipe 42 leads from the last inseries or most downstream of the tanks or compartments 32 back to themain processing tank 2. The first in series or most upstream of thetanks 32 is equipped with a mixing device 10a similar to the device ofthe main processing tank 2 and including a proportionally smallerturbine-type impeller 12a adapted to be rotatably driven through shaft14a by driving mechanism 16a suitably mounted at the top of said firstin series tank or compartment. An air or oxygen supply pipe 44 connectedto the first tank 32 has its outlet 46 disposed directly beneathimpeller 12a.

During operation of the phosphating system to apply zinc phosphatecoating to steel parts disposed in the rotating, perforate drums 6,mixing device 10 is constantly energized to effect continuous rotationof impeller-type turbine 12 and thereby agitate the bath 4 and produce aturbulent flow of bath solution within the main processingtank 2. Aparticularly strong turbulent flow of solution is produced in thedirection of flow arrows 48 in the drawing, i.e., across the base of thetank upwardly along its sides and past the heaters 8, and around andthrough the work processing drums 6. The described turbulent flowpattern is effective to almost completely eliminate any buildup of ironphosphate sludge on the base or sides of the processing tank, the drums6, and other system components submerged in the bath. Surface turbulencecreated in the bath provides for significantly increased aeration of thebath solution and the consequent precipitation of iron compounds etchedfrom the work in process out of solution in the bath. The constantagitation of the bath maintains the iron precipitate in suspension. Theprecipitated iron is continuously purged from the bath by constantlyoperating circulating pump 28 to withdraw a measured amount of bathsolution from the processing tank through intake pipe 20 and conduct thesolution through transfer pipe 30 to the first of the auxiliary tanks orcompartments 32. Solution discharged into the first auxiliary tank orcompartment flows through the tanks in a substantially quiescent state;along the flow path indicated by arrows 50 in the drawing, that is overthe sides of the adjoining tanks or compartments and beneath each of thetransverse baffles 34. During this quiescent downstream flow, the ironprecipitate settles out of solution and accumulates as sediment orsludge 52 in the funnel-shaped lower sections of the auxiliary tanks,ergo the solution which eventually arrives at the upper end of the mostdownstream tank or compartment is substantially clarified. The clarifiedsolution is discharged through return pipe 26 back to the upper regionof the processing bath 4 where it is directed by the turbulent flow pathin the main tank previously described, around and through the workprocessing drums 6. Thus, a flow of fresh bath solution is continu' allydirected to the work in process. Periodically the valves 38 are openedand the accumulated sediment 52 is forced downwardly, by the overheadliquid pressure through the ducts 36 and is carried off as solid wasteto approved land fill disposal areas.

The work load imposed on the system will dictate the extent of aerationrequired to maintain the bath at an optimum operating level and therebyproduce consistently uniform phosphate coatings. In many instances,efficient monitoring of the bath will indicate that surface aerationalone is insufficient to achieve maximum precipitation of the ironcompounds present in the bath. In such instances precipitation may beenhanced by introducing compressed air or pure oxygen into the mainprocessing tank 2 through pipe 18 and/or 24, and- /or into the first ofthe auxiliary tanks 32 through pipe 44 and simultaneously energizing thesecondary mixing device 100. Selection of the last mentioned option ineffect converts the first auxiliary tank from 'a settling tank to a backup or alternate aeration and precipitation tank and sedimentation takesplace only in the succeeding downstream auxiliary tanks or compartments32. By reason of the location of pipe outlets 20 and 46 air or oxygenintroduced into the bath solution through either pipe 18 or 24 iswhipped into dispersion in the bath solution by the respective impellersl2 and 12a. Air or oxygen introduced through pipe 24 is drawn intosolution by reason of the proximity of outlet 22 to the intake pipe 26of the circulating pump 28. In all cases the points of air or oxygenintroduction coincide with points of heavy solution turbulence. At agiven dissolved iron level, air or oxygen introduction through pipe 18,either continuously or periodically, will be sufficient to achieve themaximum iron precipitation required. When the iron compound level ishigher than that which can be efficiently precipitated by the apparatusin the main processing tank, the auxiliary or back up treatment systemmay be employed in conjunction with the main system to compensate forthe increased load. Under certain circumstances it may be advisable tocontrollably reduce the amount of iron precipitate present in the mainprocessing tank and to generate precipitate primarily between said tankand the settling tanks, in which case air or oxygen is introducedthrough pipes 24 and 44 with the auxiliary mixer 10a energized but airor oxygen is not introduced into the bath 4 through pipe 18. A selectedcombination of usage of the above described aeration means to vary theextent and location of iron precipitation affords optimum efficiency,minimum cost and maximum quality products under varying work loads wherevarying levels of dissolved iron compounds are experienced.

From the foregoing description, the reader will recognize that theimproved method of iron content control and sludge removal provides fora zinc phosphating system which can be operated with substantiallyconstant bath analysis for indefinite time periods without requiringtank dumping or decanting, or oxidizing chemical addition or batchaeration to reconstitute the bath, with an almost negligible loss ofuseful bath solution. Obviously, the elimination of continuous run offor batch dumping of chemical pollutants into local sewage treatmentsystems or drains leading directly to watercourses represents asignificant step in the direction of improved environmental qualitycontrol procedures.

Those who seek a more precise definition of the scope of the inventionprotected by these Letters Patent should now refer to the claims whichfollow.

I claim:

1. A method of controlling the iron content of a phosphating bath whichcomprises constantly agitating the bath by means of a continuouslyoperated mixing device disposed in the work processing tank to provide astrong surface turbulence in the bath and thereby to promote aeration inthe bath and precipitate dissolved iron compounds out of solution in thebath and maintain the iron precipitate in suspension in the bath,continuously purging iron precipitate from the bath by circulating bathsolution from the work processing tank through at least one auxiliarytank wherein iron precipitate settles out of suspension in the bathsolution, and continuously discharging clarified bath solution from theauxiliary tank back to the work processing tank.

2. A method according to claim 1 comprising the additional step ofperiodically purging accumulated iron bearing sediment from theauxiliary tank.

3. A method according to claim 1 including the additional step of atleast periodically introducing air or pure oxygen into the workprocessing tank at points ad- 5 jacent the mechanical mixing device.

4. A method according to claim 1 including the additional step of atleast periodically introducing air or pure oxygen into the circulatingbath solution.

5. A method of controlling the iron content of a phosphating bath whichcomprises continuous aerating and agitating of the phosphating bathsolution by forcing air or oxygen piped into the processing tank todisperse throughout the bath by means of an impeller disposed in theprocessing tank adjacent the region of air or oxygen input wherebydissolved iron is precipitated out of solution in the bath andmaintained in suspension in the bath through impeller created turbulencein the bath, continuously purging iron precipitate from the bathsolution by pumping solution from the processing tank through a seriesof auxiliary tanks wherein the precipitated iron settles out ofsolution, and continuously discharging relatively clarified solutionfrom the most downstream of said auxiliary tanks back into theprocessing tank.

6. A method according to claim 5 comprising the additional step ofperiodically purging accumulated iron sediment from said auxiliarytanks.

7. A method according to claim 5 comprising the additional steps oflocating the impeller and air or oxygen input pipe generally centrallyand in the lowermost region of the processing tank whereby highlyaerated and turbulent solution is forced outwardly and upwardly alongthe sides of the tank and thence inwardly toward the work processingregion of the tank, and locating the return from the auxiliary tanks todischarge relatively clarified solution into the uppermost region of theprocessing tank thereby maintaining a constant supply of fresh bathsolution in the work processing region.

1. A METHOD OF CONTROLLING THE IRON CONTENT OF A PHOSPHATING BOTH WHICHCOMPRISES CONSTANTLY AGITATING THE BATH BY MEANS OF A CONTINUOUSLYOPERATED MIXING DEVICE DISPOSED IN THE WORK PROCESSING TANK TO PROVIDE ASTRONG SURFACE TURBULENCE IN THE BATH AND THEREBY TO PROMOTE AERATION INTHE BATH AND PRECIPITATE DISOLVED IRON COMPOUNDS OUT OF SOLUTION IN THEBATH AND MAINTAIN THE IRON PRECIPITATE IN SUSPENSION IN THE BATH,CONTINUOUSLY PURGING IRON PRECIPITATE FROM THE BATH BY CIRCULATING BATHSOLUTION FROM THE WORK PROCESSING TANK THROUGH AT LEAST ONE AUXILIARYTANK WHEREIN IRON PRECIPITATE SETTLES OUT OF SUSPENSION IN THE BATHSOLUTION, AND CONTINUOUSLY DISCHARGING CLARIFIED BATH SOLUTION FROM THEAUXILIARY TANK BACK TO THE WORK PROCESSING TANK.
 2. A method accordingto claim 1 comprising the additional step of periodically purgingaccumulated iron bearing sediment from the auxiliary tank.
 3. A methodaccording to claim 1 including the additional step of at leastperiodically introducing air or pure oxygen into the work processingtank at points adjacent the mechanical mixing device.
 4. A methodaccording to claim 1 including the additional step of at leastperiodically introducing air or pure oxygen into the circulating bathsolution.
 5. A method of controlling the iron content of a phosphatingbath which comprises continuous aerating and agitating of thephosphating bath solution by forcing air or oxygen piped into theprocessing tank to disperse throughout the bath by means of an impellerdisposed in the processing tank adjacent the region of air or oxygeninput whereby dissolved iron is precipitated out of solution in the bathand maintained in suspension in the bath through impeller createdturbulence in the bath, continuously purging iron precipitate from thebath solution by pumping solution from the processing tank through aseries of auxiliary tanks wherein the precipitated iron settles out ofsolution, and continuously discharging relatively clarified solutionfrom the most downstream of said auxiliary tanks back into theprocessing tank.
 6. A method according to claim 5 comprising theadditional step of periodically purging accumulated iron sediment fromsaid auxiliary tanks.
 7. A method according to claim 5 comprising theadditional steps of locating the impeller and air or oxygen input pipegenerally centrally and in the lowermost region of the processing tankwhereby highly aerated and turbulent solution is forced outwardly andupwardly along the sides of the tank and thence inwardly toward the workprocessing region of the tank, and locating the return from theauxiliary tanks to discharge relatively clarified solution into theuppermost region of the processing tank thereby maintaining a constantsupply of fresh bath solution in the work processing region.